In a state of the art resonant LLC converter, a resonant capacitor and two inductors form a resonant circuit, possible in connection with additional components. The converter further includes a transformer and rectifier circuit that is used to create a DC output voltage. The output current can be made continuous by adding a series inductance in the rectifier circuit. A lot of variations exist for this type of converter, including rectification with a bridge rectifier or no rectification at all as used in a lighting application. Also the transformer can be left out in some cases.
In order to achieve satisfactory operation of a resonant converter, it is important that the switches for generating an alternating current are switched on and off at the right instant. A frequency at which the switches are operated defines a mode of operation of the converter.
The state of the art converter is usually controlled by a frequency with 50% duty cycle by a control logic circuit, where the output power of the converter is controlled by varying the operating frequency. It is also possible to use a duty cycle different from 50%. In this case also the duty cycle determines the output power.
This control principle by frequency gives some important disadvantages:                When controlled by a frequency, the voltages and currents in the resonant part at the start of each time interval (high side or low side conduction interval) are not only depending on the frequency, but also on the history of previous conduction strokes. This history gives rise to rather large transient responses and more complicated stability issues of the control loop, especially if the converter operates close to resonance.        For modern designs, efficiency at low load and input power during standby (no output power taken) have become a major issue. With the traditional 50% duty cycle control this issue is difficult to solve, because of the large circulating energy at low load. A solution for this is burst mode, but burst mode is not always allowed, because of the large ripple at the output. Also sudden changeover from normal mode to burst mode or back can give intolerable transients at the output.        Operation at low duty cycles is a solution to keep large efficiency at low power. Varying the duty cycle at a fixed frequency can however give intolerable change in sign of the loop gain.        
U.S. Pat. No. 6,711,034 discloses a resonant converter where the frequency of the switch conduction times are controlled by a timer, while asymmetry in secondary diode currents is compensated by measuring an electrical magnitude, for example the primary current.